1. Field of the Invention
The present invention relates to a laser beam recorder which applies a signal to an optical disk when fabricating the optical disk and more particularly, to a multi-focusing laser beam recorder and a multi-focusing method thereof.
2. Discussion of the Related Art
Generally, a laser beam recorder sensitizes a photoresist deposited on a glass substrate in response to data to be recorded, when fabricating an optical disk. The laser beam recorder includes a laser light source, an optical modulator for modulating the laser light in response to a signal, an optical system and a focus servo system for focusing the laser light on the photoresist deposited on the glass substrate, and a spindle for rotating the glass substrate.
A focusing method of the laser beam recorder will be described with reference to FIGS. 1 and 2.
FIG. 1 shows a focus system of a laser beam recorder according to the background art. FIG. 2 shows a focus servo system of an object lens of FIG. 1.
As shown in FIG. 1, a krypton (Kr) laser for recording a signal is to be incident upon a glass substrate 1 on which a photoresist is deposited. The incident Kr laser is reflected upon a reflecting mirror 3 through a first optical divider 2. The reflected Kr laser is to be incident upon the glass substrate 1 through an object lens 4. An He--Ne laser for focusing is to be incident upon the reflecting mirror 3 through a second optical divider 5 and the first optical divider 2 sequentially. The He--Ne laser reflected upon the reflecting mirror 3 is focused on the glass substrate 1 where the Kr laser is to be incident through the object lens 4. The He--Ne laser incident upon the glass substrate 1 is reflected again to be incident upon the first optical divider 2. The first optical divider 2 permits only 633 nm wavelength band of the He--Ne laser to be incident upon the second optical divider 5. The He--Ne laser incident upon the second optical divider 5 is to be incident upon a position sensor 6 so that it is converted to an electrical signal.
Since the position sensor 6 is divided into A area and B area, an amount of current from the A area becomes same as that from the B area if the He--Ne laser is to be incident upon the center of the position sensor 6. In this case, the current difference between the A area and the B area becomes zero. It is noted that the He--Ne laser is focused on the glass substrate.
As described in detail, as shown in FIG. 2, the He--Ne laser is to be incident upon the glass substrate 1 at "III" position after passing through the object lens 4 along "I" direction. The He--Ne laser incident upon the glass substrate 1 is then reflected in "III" direction to be incident upon the center of the position sensor 6. The current difference between the A area and the B area in the position sensor 6 is maintained at zero.
However, if the distance between the glass substrate 1 and the object lens 4 is close or far away, the He--Ne laser incident upon the position sensor 6 is moved to the A area or the B area so that the amount of current from the A area or the B area becomes larger.
For example, if the distance between the glass substrate 1 and the object lens 4 is close, i.e., if the glass substrate 1 is disposed at "II" position, the He--Ne laser incident in "I" direction is reflected in "II" direction so that the current difference between the A area and the B area in the position sensor 6 is not maintained at zero.
At this time, an OP amplifier obtains current difference between the A area and the B area in the position sensor 6 and amplifies the current difference so as to flow the current into a solenoid 7 which adjusts the position of the object lens 4.
The solenoid 7 moves the object lens 4 upwardly to maintain the distance between the glass substrate 1 and the object lens 4 at a certain interval.
As discussed above, the laser beam recorder allows focus servo in only one position (one pit width).
Since the size of the laser beam is uniformly set, the laser beam recorders required to fabricate a compact disk (CD) and a digital video disk (DVD) are different from each other. The pit width of the CD is 0.5 .mu.m approximately and that of the DVD is 0.2 .mu.m-0.3 .mu.m approximately. For this reason, the size of the laser beam focused on the glass substrate depends on the CD or the DVD.
In this respect, there is provided a laser beam recorder for use in both the CD and the DVD. This laser beam recorder is divided into a two-beam type and a beam expander type.
FIG. 3 shows a focus system of a beam expander type laser beam recorder for use in both CD and DVD. FIG. 4 shows the size of a beam focused in response to the size of a beam incident upon an object lens. FIG. 5 shows an optical system of a two-beam type laser beam recorder for use in both CD and DVD.
As shown in FIG. 3, the beam expander type laser beam recorder expands a waist portion of a Kr laser beam using a beam expander 10 in order to apply to the DVD. While, the beam expander 10 is removed from the beam expander type laser beam recorder for use in the CD.
As discussed above, if a waist portion ("A" portion in FIG. 4) of the beam to be incident upon the object lens 4 is expanded using the beam expander, a waist portion ("B" portion in FIG. 4) of the beam to be focused becomes smaller relatively.
The two-beam type laser beam recorder includes a sub-beam 21 and a main beam 22 in a fixed optical system 20. The sub-beam 21 is adjusted to be suitable for the size of the CD, while the main beam 22 is adjusted to be suitable for the size of the DVD. These sub and main beams 21 and 22 are transferred to the object lens 4 in a moving optical system 23.
In other words, when the two-beam type laser beam recorder is used for the CD, a shutter in a direction of the sub-beam 21 is opened to pass through only the sub-beam 21. When the two-beam type laser beam recorder is used for the DVD, a shutter in a direction of the main beam 21 is opened to pass through only the main beam 22.
However, the laser beam recorder and a focusing method thereof according to the background art have several problems.
Since the beam expander type laser beam recorder has each of paths of the laser beam when the beam expander is both included and removed, it is necessary to align again the optical system. This increases time consumption in the process steps and causes inconvenience to newly set a mastering condition.
The two-beam type laser beam recorder has good stability of the system but uses optical components doubly. This causes high price in the overall system. In addition, the two-beam type laser beam recorder has a problem that the laser of high power must be used.